Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. Based on the determined class of the PD, the PSE applies the required power to the PD.
In typical 802.3af PoE applications, MOSFETs are used to control application of power to the PSE output port. MOSFETs are chosen because they cause low power loss. A properly sized MOSFET may have an on-resistance (RON) below 0.5Ω and thus causes a power loss such as 60 mW. In many 802.3af-compliant PSE implementations, sense resistor RSENSE used for sensing port current causes more loss than the MOSFET.
However, the IEEE 802.3af standard requires a PSE to monitor the link for the Maintain Power Signature (MPS) from a PD in order to remove power supplied to the PD if the MPS is absent for more than a predefined time period. Conventional circuitry combined with the MOSFET for supporting this function causes a substantial power loss.
The MPS consists of two components—an AC MPS component and a DC MPS component. A PSE may optionally monitor the AC MPS component only, the DC MPS component only, or both the AC and the DC MPS components. A PSE considers the AC MPS component to be present when it detects an AC impedance at its output port equal to or lower than 27 kΩ. A PSE considers the AC MPS component to be absent when it detects an AC impedance at its output port greater than 27 kΩ. A PSE should remove power from its output port when the AC MPS component has been absent for a time duration greater than the MPS drop out time limit, which is in the range from 300 ms to 400 ms.
The IEEE 802.3af standard requires a PSE that monitors the AC MPS component to meet certain parameter requirements. In order to comply with these requirements, a PSE with an AC disconnect-detection function conventionally has a diode in series with the MOSFET. The purpose of the diode is to provide the PSE output port with a high impedance when the port voltage is greater than the PSE power supply voltage. The diode achieves this goal by having a high impedance when reversed biased.
When the PSE supplies power to its output port, the diode is forward biased. In the forward biased condition, power loss in the diode is defined by VBE×IPORT, where IPORT is the output current, and VBE is the voltage across the diode equal to 0.7V at 25° C. and to 0.6V at 75° C. As a result, the power loss may be in the range from 200 mW to 250 mW depending on temperature. Hence, the diode causes far more power loss than the MOSFET used for switching power to the port.
Therefore, it would be desirable to provide a PSE with a power-efficient output port arrangement that does not use a diode for supporting an AC disconnect-detect function.